Composition and method for surface treatment of oxidized metal

ABSTRACT

The present invention provides an electrically conductive element for a proton exchange membrane fuel cell having low electrical contact resistance and high corrosion resistance. The conductive element comprises a corrosion susceptible metal substrate with a surface, which is preferably treated to activate the surface (i.e. to remove a passivation layer of oxides from the surface) with an acidic treatment solution. The treated surface is then overlaid with an electrically conductive, corrosion-resistant, protective coating to protect the substrate re-forming a passivation layer while exposed to the corrosive environment of the fuel cell. The present invention also provides methods of preparing an electrically conductive element to have low electrical contact resistance and high corrosion resistance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/638,917 filed on Aug. 11, 2003 now abandoned , the entire disclosureof which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to compositions and methods for surfacetreatment of oxidized metal and more particularly using suchcompositions and methods to prepare corrosion-resistant electricallyconductive elements.

BACKGROUND OF THE INVENTION

Fuel cells have been proposed as a power source for electric vehiclesand other applications. One known fuel cell is the PEM (i.e., ProtonExchange Membrane) fuel cell that includes a so-called MEA(“membrane-electrode-assembly”) comprising a thin, solid polymermembrane-electrolyte having an anode on one face and a cathode on theopposite face. The anode and cathode typically comprise finely dividedcarbon particles, very finely divided catalytic particles supported onthe internal and external surfaces of the carbon particles, and protonconductive material intermingled with the catalytic and carbonparticles. The MEA is sandwiched between a pair of electricallyconductive contact elements which serve as current collectors for theanode and cathode, which may contain appropriate channels and openingstherein for distributing the fuel cell's gaseous reactants (i.e., H₂ andO₂/air) over the surfaces of the respective anode and cathode.

Bipolar PEM fuel cells comprise a plurality of the MEAs stacked togetherin electrical series while being separated one from the next by animpermeable, electrically conductive contact element known as a bipolarplate or current collector. The current collector or bipolar plate hastwo working surfaces, one confronting the anode of one cell and theother confronting the cathode on the next adjacent cell in the stack,and electrically conducts current between the adjacent cells. Contactelements at the ends of the stack contact only the end cells and arereferred to as end plates.

Contact elements are often constructed from electrically conductivemetallic materials. In an H₂ and O₂/air PEM fuel cell environment, thebipolar plates and other contact elements (e.g., end plates) are inconstant contact with moderately acidic solutions (pH 3-5) and operatein a highly oxidizing environment, being polarized to a maximum of about+1 V (vs. the normal hydrogen electrode). On the cathode side thecontact elements are exposed to pressurized air, and on the anode sideexposed to atmospheric hydrogen. Unfortunately, many metals aresusceptible to corrosion in the hostile PEM fuel cell environment, andcontact elements made therefrom either dissolve (e.g., in the case ofaluminum), or form highly electrically resistive, passivating oxidefilms on their surface (e.g., in the case of titanium or stainlesssteel) that increases the internal resistance of the fuel cell andreduces its performance. Thus, there is a need to provide electricallyconductive elements that maintain electrical conductivity, resist thefuel cell aggressive environment, and improve overall operationalefficiency of a fuel cell.

SUMMARY OF THE INVENTION

The present invention relates to metallic substrates such as anelectrically conductive element which in one preferred embodimentcomprises an electrically conductive corrosion susceptible metalsubstrate having a contact resistance of less than or equal to about 10mΩ-cm² under a compressive force of about 2700 kPa, and an electricallyconductive corrosion resistant coating overlying one or more regions ofa surface of the metal substrate.

Other preferred embodiments according to the present invention includemethods of treating an oxidized metal substrate such as when making anelectrically conductive element. One method comprises providing aconductive metal substrate having a surface susceptible to passivationin the presence of oxygen by forming oxides. An acidic treatmentsolution is applied to the surface to remove the oxides and reducecontact resistance of the surface as compared to its pre-treated state.The surface is rinsed to remove substantially all of the treatmentsolution, and then coated with a corrosion resistant electricallyconductive coating.

Alternate preferred embodiments of the present invention contemplate acomposition for treating a surface of a metallic substrate having ametal oxide at the surface. The composition comprises a solutioncomprising a solvent and a solute, where the solution has a pH equal toor less than about 4, and where the solute comprises an anionic speciesof one or more halogen ions present in the solution at a concentrationof from about 1×10⁻¹ to about 1×10⁻⁶ molarity.

Other alternate embodiments of the present invention include methods oftreating a surface of a metallic substrate having metal oxide at thesurface, the method comprising contacting the surface with an acidicsolution to react with the metal oxide without an impressed electricalcurrent to form a metal halide species soluble in the solution where thesolution has a pH of from 0 to about 4. The metal halide is separatedfrom the surface. The surface is rinsed with a rinse solution, whichthus, removes the solution and the metal halide from the surface.

Further the present invention contemplates in other preferred alternateembodiments a method of treating a surface of a metallic substratehaving a metal oxide at the surface by contacting the surface with anacidic solution to react with the metal oxide without an impressedelectrical current to form a metal halide species soluble in thesolution wherein the solution has an anionic species of one or morehalogen ions present in the solution at a concentration of from about1×10⁻¹ to about 1×10⁻⁶ molarity. The metal halide is separated from thesurface and the surface is rinsed with a rinse solution, thus removingthe solution and the metal halide from the surface.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic, exploded, isometric, illustration of aliquid-cooled PEM fuel cell stack (only two cells shown);

FIG. 2 is an exploded, isometric view of a bipolar plate useful with PEMfuel cell stacks like that illustrated in FIG. 1;

FIG. 3 is a partial cross-sectional view in the direction 3-3 of FIG. 2;

FIG. 4 is a terminal collector end plate;

FIG. 5 is a magnified portion of the bipolar plate of FIG. 3;

FIG. 6 is an exemplary testing apparatus used to measure the contactresistance of a sample; and

FIG. 7 is a graph comparing the electrical contact resistance of 316 Lstainless steel samples at various applied pressure values preparedaccording to the present invention and by prior art methods ofpreparation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

The present invention relates to composition and method for surfacetreatment of oxidized metal and more particularly to such as applied topreparation of corrosion-resistant electrically conductive elements.

The present invention relates to a PEM fuel cell having at least onecell comprising a pair of opposite polarity electrodes, a membraneelectrolyte interjacent the electrodes for conducting ions therebetween,and an electrically conductive element confronting at least one of theelectrodes. The conductive element has a working surface that serves toconduct electrical current from that electrode. The conductive elementcomprises a metal substrate with a surface, which is preferably treatedaccording to the present invention, to activate the surface, that is, toremove a passivation or oxide layer from the surface. The treatedsurface is then overlaid with an electrically conductive,corrosion-resistant, protective coating to protect the substrate fromthe corrosive environment of the fuel cell. By “corrosion susceptiblemetal” is meant that the metal is either oxidized/passivated ordissolved by the cell's environment.

To gain a better understanding of the present invention, an exemplaryfuel cell where the present invention may be employed is shown in FIG.1, which depicts two individual proton exchange membrane (PEM) fuelcells connected to form a stack having a pair ofmembrane-electrode-assemblies (MEAS) 4, 6 separated from each other byan electrically conductive, liquid-cooled, bipolar separator plateconductive element 8. An individual fuel cell, which is not connected inseries within a stack, has a separator plate 8 with a singleelectrically active side. In a stack, a preferred bipolar separatorplate 8 typically has two electrically active sides 20, 21 within thestack, each active side 20, 21 respectively facing a separate MEA 4, 6with opposite charges that are separated, hence the so-called “bipolar”plate. As described herein, the fuel cell stack is described as havingconductive bipolar plates; however the present invention is equallyapplicable to stacks having only a single fuel cell.

The MEAs 4,6 and bipolar plate 8 are stacked together between stainlesssteel clamping terminal plates 10,12 and end contact fluid distributionelements 14,16. The end fluid distribution elements 14, 16, as well asboth working faces or sides 20,21 of the bipolar plate 8, contain aplurality of lands adjacent to grooves or channels on the active faces18, 19, 20, 21, 22, and 23 for distributing fuel and oxidant gases(i.e., H₂ and O₂) to the MEAs 4,6. Nonconductive gaskets or seals 26,28, 30, 32, 33, and 35 provide seals and electrical insulation betweenthe several components of the fuel cell stack. Gas-permeable conductivediffusion media 34, 36, 38, and 40 press up against the electrode facesof the MEAs 4, 6. Additional layers of conductive media 43, 45 areplaced between the end contact fluid distribution elements 14, 16 andthe terminal collector plates 10, 12 to provide a conductive pathwaytherebetween when the stack is compressed during normal operatingconditions. The end contact fluid distribution elements 14, 16 press upagainst the diffusion media 34, 43 and 40, 45 respectively.

Oxygen is supplied to the cathode side of the fuel cell stack fromstorage tank 46 via appropriate supply plumbing 42, while hydrogen issupplied to the anode side of the fuel cell from storage tank 48, viaappropriate supply plumbing 44. Alternatively, air may be supplied tothe cathode side from the ambient, and hydrogen to the anode from amethanol or gasoline reformer, or the like. Exhaust plumbing 41 for boththe H₂ and O₂/air sides of the MEAs is also provided. Additionalplumbing 50 is provided for circulating coolant from a storage area 52through the bipolar plate 8 and end plates 14,16 and out the exitplumbing 54.

FIG. 2 is an isometric, exploded view of a bipolar plate 56 comprising afirst exterior metal sheet 58, a second exterior metal sheet 60, and aninterior spacer metal sheet 62 interjacent the first metal sheet 58 andthe second metal sheet 60. The exterior metal sheets 58, 60 are made asthin as possible (e.g., about 0.002-0.02 inches thick), which may beformed by stamping, by photo etching (i.e., through a photolithographicmask), electroforming, or any other conventional process for shapingsheet metal. The external sheet 58 has a first working surface 59 on theoutside thereof which confronts a membrane-electrode-assembly (notshown) and is formed so as to provide a plurality of lands 64 whichdefine therebetween a plurality of grooves 66 known as a “flow field”through which the fuel cell's reactant gases (i.e., H₂ or O₂) flow in atortuous path from one side 68 of the bipolar plate to the other side 70thereof. When the fuel cell is fully assembled, the lands 64 pressagainst the carbon/graphite papers (such as 36 or 38 in FIG. 1) which,in turn, press against the MEAs (such as 4 or 6 in FIG. 1,respectively). For drafting simplicity, FIG. 2 depicts only two arraysof lands 64 and grooves 66. In reality, the lands and grooves 64, 66will cover the entire external surfaces of the metal sheets 58, 60 thatengage the carbon/graphite papers. The reactant gas is supplied togrooves 66 from a header or manifold groove 72 that lies along one side68 of the fuel cell, and exits the grooves 66 via anotherheader/manifold groove 74 that lies adjacent the opposite side 70 of thefuel cell.

As best shown in FIG. 3, the underside of the sheet 58 includes aplurality of ridges 76 which define therebetween a plurality of channels78 through which coolant passes during the operation of the fuel cell.As shown in FIG. 3, a coolant channel 78 underlies each land 64 while areactant gas groove 66 underlies each ridge 76. Alternatively, the sheet58 could be flat and the flow field formed in a separate sheet ofmaterial. Metal sheet 60 is similar to sheet 58. The internal surface 61(i.e., coolant side) of sheet 60 is shown in FIG. 2.

In this regard, there is depicted a plurality of ridges 80 definingtherebetween a plurality of channels 82 through which coolant flows fromone side 69 of the bipolar plate to the other 71. Like sheet 58 and asbest shown in FIG. 3, the external side of the sheet 60 has a workingsurface 63 having a plurality of lands 84 thereon defining a pluralityof grooves 86 through which the reactant gases pass. An interior metalspacer sheet 62 is positioned interjacent the exterior sheets 58, 60 andincludes a plurality of apertures 88 therein to permit coolant to flowbetween the channels 82 in sheet 60 and the channels 78 in the sheet 58thereby breaking laminar boundary layers and affording turbulence whichenhances heat exchange with inside surfaces 90, 92 of the exteriorsheets 58, 60 respectively.

The spacer sheet 62 is positioned between the first sheet 58 and secondsheet 60, where the ridges 76 on the first sheet 58 and the ridges 80 onthe second sheet 60 are bonded (e.g. by a bonding layer 85, such asbrazement or adhesives) to the spacer sheet 62. As recognized by one ofskill in the art, the current collectors of the present invention mayvary in design from those described above, such as for example, in theconfiguration of flow fields, placement and number of fluid deliverymanifolds, and the coolant circulation system, however, the function ofconductance of electrical current through the surface and body of thecurrent collector functions similarly between all designs.

The present invention is also applicable to other conductive elements ina fuel cell, such as terminal collector end plates like the exemplaryone shown in FIG. 4. A terminal collector end plate 99 (such as 10 or 12of FIG. 1) has an electrically non-conductive region 100, as well as anelectrically conductive region 102. The conductive regions 102 of theterminal plate 99 are typically separated from the non-conductive region100 by sealing gaskets 33, 35 (FIG. 1). Apertures 104 within thenon-conductive region 100 extend through the body, or substrate, 128 ofthe terminal plate 99 and permit fluid transport (e.g. H₂, O₂, coolant,anode and cathode effluent) both into and out of the stack duringoperating conditions. The particular quantity or sequence of theapertures 104 is not limiting, and is merely exemplary as describedherein, as numerous configurations are possible as recognized by one ofskill in the art. A bipolar plate flow field design may dictate theinlet and outlet aperture 104 configurations and fluid deliveryplacement. An electrically conductive collector tab 120 can be attachedto external leads, facilitating the external collection of current fromthe stack.

Selection of the material of construction for an electrically conductiveelement within a fuel cell, such as bipolar plates or terminal plates,includes weighing such parameters as overall density (mass and volume),electrical contact resistance of the substrate measured at the surface,bulk conductivity, and corrosion and oxidation resistance. Thus, theimportant considerations for an electrical element include surface andbulk intrinsic conductivity of the material to perform as an electricalcurrent collector, while withstanding the potentially corrosiveconditions experienced within the fuel cell. It is preferred thatcurrent collectors according to the present invention comprise aconductive metal.

Metal materials undergo a variety of chemical reactions that impactfunctionality within the fuel cell. Passivation, as used herein,generally refers to a conversion process of treating a metal to renderthe surface less chemically reactive and falls within the process ofcorrosion, where the metal is attacked by a corrosive agent. Afterpassivation has occurred in certain metals, the surface is coated with aprotective passivation film that renders the surface of the plate moreelectrochemically inert than its pre-treated state. Such a passivationlayer may protect the underlying metal by making it less prone tocorrosion when compared with the pre-treated metal surface, such as isthe case with stainless steel alloys.

Although metal surfaces are often intentionally processed to form thepassivation layer, mere exposure to oxygen (or air) also passivatescertain metallic surfaces. While having corrosion resistance benefits,passivated surfaces in conductive metals also exhibit high electricalcontact resistance values, generally making such metals havingpassivated surfaces poor electrical conductors. Although not limiting tothe manner in which the present invention operates, it is believed thatone of the primary reasons for poor electrical surface conductivityafter passivation of a metal surface is due to the formation of metaloxides in a passivation layer. Activation, in contrast to passivation,is the conversion process of making a surface relatively more active.Thus, activation often entails removing or thinning the passivationlayer from a surface, by subjecting the surface to reducing conditions,which renders it more electrochemically active and hence reduceselectrical contact resistance.

Stainless steel is generally defined as an iron-chromium alloy with aminimum of 9% chromium. Other ferritic, martensitic, or austeniticalloys are contemplated for use in PEM fuel cells. Stainless steels richin chromium (i.e., at least 16% by weight), nickel (i.e., at least 20%by weight), and molybdenum (i.e., at least 3% by weight) areparticularly desirable metals for use within a fuel cell, due to theirrelatively high bulk electrical conductivity and corrosion resistanceprovided by a dense passivation (i.e., metal oxide) layer at thesurface. Thin stainless steel plates can be used to decrease thevolumetric and weight power density of the fuel cell stack. Further,stainless steel materials have relatively high strength, physicaldurability, adherence to protective coatings, and are less expensivethan many other conductive metal alternatives. However, the oxide layerat the surface impermissibly increases electrical contact resistance ofthe substrate, which has previously prevented its independent use as anelectrical contact element or current collector. Further many otherrelatively lightweight metals are susceptible to corrosive attack (e.g.aluminum and titanium), and in light of such corrosion sensitivity andsimilar propensity for oxidation, various protective coatings are usedfor the metal substrate. Often such protective coatings increase theelectrical resistance of the metal plate to unacceptable levels or arevery costly, such as with gold or platinum coatings. Thus, there is atrade-off between conductivity and corrosion protection. It is an objectof the present invention to reduce the overall electrical contactresistance of an electrically conductive element, while providingcorrosion and oxidation resistance for the underlying metal substrate ofthe conductive element.

Previous methods to overcome such high electrical contact resistanceinclude treating a conductive metal substrate to clean the surface of apassivation layer (e.g. metal oxides), and then coating with aprophylactic coating. Current cleaning methods often employ cathodiccleaning where electrical current (e.g. a current density of 4 A/cm²) isimpressed onto the conductive substrate which is in contact with anelectrolyte to facilitate the generation of gas bubbles at the surface,such as cathodic cleaning described in ASTM B254 7.4.1, for example.Such cathodic cleaning generally takes a minimum processing time of 10minutes (typically much longer) to effectively clean the metalsubstrate, and is not generally suitable for high production coilcoating processes. Reducing processing time is an importantconsideration when using continuous manufacturing methods for creatingelectrically conductive elements or other components within anelectrochemical cell.

Other cleaning methods include mechanical abrasion of the surface, orcleaning the substrate with commercially available alkaline cleaners,acidic solvents, or pickle liquors. Other methods of treating aconductive metal substrate include pre-sputtering the surface of themetal in a physical vapor deposition (PVD) chamber, and radio-frequencyglow-discharge treatment. The above described methods of removing thepassivation layer from the metal are not thought to sufficiently reducecontact resistance to an acceptable value and simultaneously reduceprocessing time, to allow for the use of metals susceptible topassivation as current collector elements.

According to one aspect of the present invention, an electricallyconductive element or current collector made of a corrosion-susceptiblemetal is treated to impart a low contact resistance and the ability towithstand corrosion and oxidative attack, while minimizing the amount ofsubstrate metal lost during the activation process. Such a treatmentpermits the use of metals, such as stainless steel, which previously hadtoo high of an electrical contact resistance for practical use in a fuelcell.

Corrosion-susceptible metal substrates treated according to the presentinvention have significantly reduced contact resistance values ascompared with their pre-treated state. In one preferred embodiment ofthe present invention, a surface of the electrically conductive elementis activated by treatment with an acidic solution having a compositionsuch that metal oxides in a passivation layer at the surface are solubletherein, and thus are removed from the surface of the metal substrate.The acid solution is designed to react with and convert metal oxides tosoluble constituents that may be readily removed from the surface. Inselecting the preferred composition of the acid solution, thethermodynamics for various constituents in the metal substrate areevaluated over a range of pH values. Thus, for a preferred embodiment,the metal substrate is stainless steel, such as for example 316L (UNSS31603), an alloy which contains iron, chromium, nickel, and molybdenum.By evaluating the thermodynamics of each constituent metal oxide, itbecomes possible to select a desirable pH range where the metal oxidephases might form unstable species. While evaluating such thermodynamicdata, certain preferable anions have been discovered which alter theconstituent metal oxide phases at preferred conditions, thus creatingsoluble metal species that react with the anions, thus dissolving themetal oxide phases by ionic interaction at the metal surface.

In preferred embodiments of the present invention, the pH of the acidicsolution is less than 7, preferably between 0 and about 4, and mostpreferably between about 2 to about 4. The acidic solution comprises asolvent and a solute. The solute preferably comprises at least oneacidic anion and a corresponding conjugate cation. As recognized by oneof skill in the art, the selection of anion(s)/cation(s) for the acidicsolution affects the pH (due to the pK_(a) value of the anion andcation). Thus, anions that provide the requisite pH, while also enablingformation of soluble metal species from the metal oxides are preferredfor use with the present invention.

Preferred anions according to the present invention are halogen ions,including fluoride (F⁻), chloride (Cl⁻), bromide (Br⁻), iodide (I⁻), aswell as sulfate (SO₄ ²⁻), nitrate (NO₃ ⁻), and mixtures thereof. Themost preferred anions according to the present invention are fluoride(F⁻) and sulfate (SO₄ ²⁻). The preferred cations comprise hydrogenprotons. Preferred solvents in the acidic solution according to thepresent invention comprise water. Thus, preferred solutes (with bothanions and cations) according to the present invention include halides,such as hydrofluoric acid, hydrochloric acid, hydrobromic acid,hydriodic acid, as well as, sulfuric acid, nitric acid, and mixturesthereof.

As an example, when the electrically conductive element is selected tobe 316L stainless steel (a corrosion-susceptible metal substrate), thenickel and ferric oxides have limited domains of stability across therange of all pH values, and will react to form other species at low pHvalues and high potentials. However, the chromium oxide species arestable and exist even at very low pHs. It has been found that when afluoride anion is incorporated at concentrations of greater than 1×10⁻⁶into an acidic treatment solution (preferably in a solution alsocomprising sulfuric acid that has a pH equal to or below 4) the chromicoxides will form chromium fluoride. Chromium fluoride is soluble in theacidic treatment solution. The metal oxides are thus transformed byreaction with the anions to metal compounds that are soluble in thesolvent at the appropriate pH conditions, such as metal halides when theanion is a halogen.

In one preferred embodiment of the present invention, where the halogenion in the solute is fluoride, the preferred concentration of fluorideanions in the acidic solution of the present invention is from about1×10⁻¹ to about 1×10⁻⁶ molarity. This concentration is selected tooptimize the pH of the solution and the amount of anions needed toachieve the necessary reaction with metal oxides at the surface. Fromempirical observation, the greater the concentration of preferred anions(i.e., closer to 1×10⁻¹), the higher the pH value may be while stillachieving the necessary reduction in metal oxides at the surface.Likewise, it has been observed that the less the concentration ofpreferred anions (i.e., closer to 1×10⁻⁶) in the acid treatmentsolution, the lower the pH value must be. It should be noted that othersolutes may be present to achieve the necessary low pH value, but maynot contain anions according to the present invention that react withmetal oxides to create the desirable soluble metal constituent at thesurface of the metal substrate. In certain preferred embodiments of thepresent invention, the concentration of the halide is from between about1×10⁻³ and about 1×10⁻⁵ molarity in the acidic treatment solution, whichhas a pH of between about 2 and about 4.

Preferred temperature conditions for applying the acid treatmentsolution to the metal surface range from about 25° C. (room temperature)to about 100° C. at ambient atmospheric pressure. The acid treatmentsolutions of the present invention are not only highly effective insurface activation, but also significantly reduce the treatment time toachieve the activation. Shorter processing times prevent excessive andunnecessary removal of the underlying substrate metal, and further makethe present invention well suited for continuous manufacturingprocesses. The preferred duration for exposing the metal surface to theacid solution ranges from approximately 3 seconds up to about 600seconds (10 minutes). The most preferred duration of treatment isbetween about 3 to about 120 seconds (2 minutes). As appreciated by oneof skill in the art, the processing time is dependent on a variety offactors, including the aggressiveness of the acidic treatment solution,the character and extent of the passivation layer on the metal, and therelative amount of surface area to be activated. The present inventionis readily adaptable to continuous manufacturing processes, whichprovides increased productivity by requiring less processing time, whileenhancing the overall quality of surface activation.

Methods of treating the metal substrate according to preferredembodiments of the present invention comprise providing a conductivemetal substrate with a surface susceptible to passivation in thepresence of oxygen (i.e., corrosion-susceptible). The acidic treatmentsolution is applied to the conductive metal substrate. Such applicationof treatment solution may include submerging the metal substrate in acontainer filled with acidic treatment solution for the requisite lengthof time necessary to remove the metal oxides. In alternate preferredembodiments, the application of treatment solution may include sprayingthe conductive metal substrate with acidic treatment solution. Thesurface of the metal may also be sprayed and submerged, eithersimultaneously or in successive processing steps. The application of theacidic treatment solution ensures that the passivation layer of metaloxides is removed from the surface of the substrate. The removal of thepassivation layer provides reduced electrical contact resistance of thesubstrate when compared to the pre-treated state of the substrate.

After the acidic treatment solution has been applied, the metal oxidesare transformed by ionic interaction with the preferred anions of theacidic treatment solution to form metal species soluble in the solventof the acidic treatment solution. One preferred solvent in the acidictreatment solution according to the present invention is water. Thus,the preferred anions preferably create a metal anion species that ishydrophilic and ionic, which is soluble in water. Hence, during theapplication process, the soluble metal species is likewise dissolved insolvent and removed from the metal surface and dispersed into the acidictreatment solution.

The surface of the metal substrate is preferably rinsed after applyingthe treatment solution. The rinsing of the metal substrate surfaceremoves substantially all of any residual acidic treatment solutionremaining on the metal substrate. By “substantially all” it is meantthat a large portion of the acidic treatment solution is removed, sothat the metal surface is not detrimentally impacted by the presence ofresidual acidic treatment solution. Long term exposure to residualacidic treatment solution may cause physical deformities in the metalsurface, such as pitting or embrittlement, for example, or may impedethe efficacy of subsequent processing. Thus, it is preferred that themetal surface is rinsed to remove most, or substantially all, of theresidual acidic treatment solution.

The rinsing is preferably conducted with a similar solvent as that usedin the acidic treatment solution, which is preferably deionized water.After exposure to a low pH acid (i.e., the acidic treatment solution)during application, it is preferred that a first rinse solution ismildly acidic rather than neutral (i.e., has a pH of less than 7 andgreater than about 4) to prevent shocking of the metal surface, whichcould cause precipitates to form. In certain preferred embodiments, useof the first rinse solution is sufficient to remove substantially all ofany residual acidic treatment solution on the surface. The first rinsesolution may be used for multiple rinse sequences, if necessary. Thefirst rinse solution preferably comprises deionized water and a mild andinexpensive acid solute, such as acetic acid, carbonic acid, and thelike, or very low concentrations of more aggressive acids, such assulfuric acid.

In alternate preferred embodiments, an additional sequential rinsingstep may be employed with a second rinse solution. The second rinsesolution is preferably neutral with a pH of approximately 7, which canbe used after exposure to the first mildly acidic rinse, so that thereis no danger of shocking the metal surface. A second neutral rinsesolution preferably comprises deionized water. Each of the respectivefirst and second rinse solutions may be used multiple times to rinse thesurface of the metal substrate, if necessary, to remove substantiallyall of the acidic treatment solution.

Thus, the treatment of the surface of the metal substrate with an acidicsolution activates the surface by transforming metal oxides to solublemetal anion species. The rinsing of the acidic solution and metal anionspecies from the surface according to the present invention furtherensures removal of the passivation layer. The present invention provideselectrically conductive elements where the contact resistance of themetal is drastically reduced from its pre-treated state to a level suchthat metal prone to forming electrically insulating passivation layersof metal oxide, such as stainless steel, may be activated to the extentthat they may be employed as a metal substrate for an electricallyconductive element in a fuel cell.

In certain preferred alternate embodiments of the present invention, themetal substrate is pre-cleaned prior to applying the acidic treatmentsolution. Such cleaning typically serves to remove any loosely adheredcontaminants, such as oils, grease, waxy solids, particles (includingmetallic particles, carbon particles, dust, and dirt), silica, scale,and mixtures thereof. Many contaminants are added during themanufacturing of the metal material, and may also accumulate on thesurface during transport or storage. Thus, pre-cleaning is preferred incircumstances where the metal substrate provided for processing issoiled with contaminants. Pre-cleaning may entail mechanical abrasion;cleaning with traditional alkaline cleaners, surfactants, mild acidwashes; or ultrasonic cleaning. The choice of the appropriate cleaningprocess or sequence of cleaning processes is selected based upon boththe nature of the contaminant and the metal.

Mechanical cleaning or polishing may include abrading the surface with apad or roller comprising abrasive particles. Examples of abrasive padsmay include those having silicon carbide or aluminum oxide dispersed ona nylon matrix, or paper with sand or diamond particles, for example. Acommercially available abrasive pad having SiC on a nylon matrix isSCOTCH-BRITE® manufactured by 3M Corporation of St. Paul, Minn. Othermethods of mechanically cleaning may include abrasive blasting of thesurface, which can be done with plastic (polymer) blast beads, walnutshells, sand, or glass particles. Other mechanical cleaning or polishingmethods known to those of skill in the art may also be used.

Alkaline cleaners are preferred for pre-cleaning, especially to removeorganic oils and materials from the surface of the metal. Preferredalkaline cleaners have a pH in the range of about 9 to 14. The alkalinecleaners preferred for the present invention generally comprise abuilder composition and a surfactant to facilitate emulsification oforganic compounds. Other additives may also be included in an alkalinecleaner, including organic or inorganic additives, chelating agents, andsequestrants which enhance cleaning of the surface. Alkaline cleanersare typically applied by immersion or spray. The cleaning step isusually followed by a rinse with deionized water, prior to treating withthe acidic treatment solution.

Alkaline cleaners may operate by three mechanisms, includingsaponification, emulsificiation, or dispersion. Often all threemechanisms are employed in one particular alkaline cleaner. The buildercomposition is generally the alkaline salt, and may comprise blends ofalkaline salts, usually selected from the group consisting of: alkalimetal orthophosphates, alkali metal condensed phosphates, alkali metalhydroxides, alkali metal silicates, alkali metal carbonates, alkalimetal bicarbonates, and alkali metal borates. Preferred alkali metalsfor the builder composition are sodium and potassium. Surfactants arepreferably organic compounds that provide detergency, emulsification andwetting in an alkaline cleaner, and are well known in the art. In onepreferred alkaline cleaner, the builder constituents are potassiumhydroxide and tetrapotassium pyrophosphate, present in a water solventfrom about 10-30 wt. % and 1-10 wt. %, respectively. Such an alkalinecleaner is commercially available as PARCO® Clean 1200 from HenkelSurface Technologies of Madison Heights, Mich. Other surface cleaningmethods may include ultrasonic cleaning (generally coupled with eitheran alkaline or acidic cleaner) where electrical energy is converted bytransducers to ultrasonic waves; corona discharge or radio-frequencyglow-discharge, both of which treat the target surface to an electricaldischarge (i.e., corona) which disperses reactive oxygen or other gasmolecules to activate the surface. All of the above cleaning methods aresuitable for the pre-cleaning process.

As previously discussed, the acidic treatment solution applicationactivates the surface of an electrically conductive element substrate byremoving the passivation layer (metal oxides). However, although theunderlying metal substrate is restored to its pre-passivated state withan activated surface, the metal substrate still remains susceptible tonew corrosion where a new passivation layer comprising metal oxides mayre-form at the surface. The passivation layer will form upon exposure toa corrosion agent, such as oxygen, present in the ambient atmosphere andwithin the fuel cell. Thus, removing the passivation layer promoteselectrical conductivity by reducing overall contact resistance. However,if the metal substrate is exposed to oxygen, the passivation layer willre-form at the surface, counteracting the activation process.

Hence, after the passivation layer is removed, the electricallyconductive element is preferably protected from furthercorrosion/passivation by physically protecting it from corrosive agentsto prevent the re-formation of a passivation layer at the surface of themetal substrate. As shown in FIG. 5, the electrically conductive element(e.g. bipolar plate 56) has a protective coating 130 overlaying thecorrosion susceptible metal substrate 132 on both the first surface 59of the first sheet 58 and the second surface 63 of the second sheet 60.Further, the protective coating 130 is applied to the interior firstsurface 90 of the first sheet 58 and second surface 92 of the secondsheet 60, to protect the substrate 132 from corrosive oxidative attackby the coolant. From a practical point of view, it is not necessary tocoat the interior or coolant passages of the bipolar plate in the caseof stainless steel or titanium applications. The protective coating 130is a practical way to isolate the electrically conductive element fromcorrosive agents that are prevalent both in the processing environmentand in the fuel cell itself. Thus, it is preferred that the protectivecoating 130 is applied to the conductive and corrosion-susceptibleregions (e.g. surfaces 59, 60, 90, 92) of the electrically conductiveelement 56, such that the underlying metal substrate 132 is protectedfrom corrosive agents that would react with the metal to electricallyinactivate/passivate each surface. Thus, select regions may include onlythe electrically conductive areas that form conductive pathways acrossthe electrically conductive element or such regions may coincide to theentire surface of the substrate.

According to preferred embodiments of the present invention, theprotective coating 130 is applied within a short duration of time afterthe metal substrate 132 is removed from the rinsing process, to minimizethe re-formation of metal oxides in a passivation layer at the surface.Thus, it is preferred that the protective coating 130 is applied withina period of two hours, and most preferable that the protective coating130 is applied to the metal substrate 132 surface in 30 minutes or less.A short time between the treatment with the acidic solution and theapplication of the protective coating 130 minimizes the exposure tooxygen to reduce the amount of metal oxides or thickness of apassivation layer forming at the surface. As appreciated by one of skillin the art, various processing conditions and material characteristicsmay impact the rate of passivation at the surface, and hence may impacthow quickly the protective coating 130 should be applied to the metalsurface. Ideal processing times may be discerned based on materialcharacteristics, physical testing, and other empirical results.

The protective coatings 130 of the present invention are preferablycorrosion resistant electrically conductive coatings that protect theunderlying metal substrate 132 from exposure to corrosion agents. Morespecifically, the protective coatings 130 preferably have a resistanceless than about 50 ohm-cm² (Ω-cm²) and comprise a plurality ofoxidation-resistant, acid-insoluble, conductive particles (i.e., on theorder of about 50 microns or less than about 50 microns) dispersedthroughout an acid-resistant, oxidation-resistant polymer matrix, wherethe polymer binds the particles together and holds them on the surfaceof the metal substrate 132. The coating contains sufficient conductivefiller particles to produce a total resistance no greater than about 50ohm-cm² and has a thickness between about 2 microns and about 75microns, preferably between 2 and 30 microns, depending on thecomposition, resistivity and integrity of the coating. Thinner coatings(i.e., about 15-25 microns) are most preferred for minimizing the IRdrop through the stack. Impervious protective coatings 130 are preferredfor the present invention to protect the underlying metal substrate 132surface from permeation of corrosive agents.

Preferably, the conductive filler particles are selected from the groupconsisting of gold, platinum, graphite, carbon, nickel, conductive metalborides, nitrides and carbides (e.g. titanium nitride, titanium carbide,titanium diboride), titanium alloyed with chromium and/or palladium,niobium, rhodium, rare earth metals, and other noble metals. Mostpreferably, the particles will comprise carbon or graphite (i.e.,hexagonally crystallized carbon). The particles comprise varying weightpercentages of the coating depending on the density and conductivity ofthe particles (i.e., particles having a high conductivity and lowdensity can be used in lower weight percentages). Carbon/graphitecontaining coatings will typically contain 25 percent by weightcarbon/graphite particles. The polymer matrix comprises anywater-insoluble polymer that can be formed into a thin adherent film andthat can withstand the hostile oxidative and acidic environment of thefuel cell. Hence, such polymers, as epoxies, silicones,polyamide-imides, polyether-imides, polyphenols, fluro-elastomers,polyesters, phenoxy-phenolics, epoxide-phenolics, acrylics, andurethanes, inter alia are seen to be useful with the present invention.Both thermoset and thermoplastic polymers are suitable for producingimpermeable coatings.

In accordance with one embodiment of the invention, the conductivepolymer coating 130 is applied directly to the substrate metal 132 andallowed to dry/cure thereon. The coating 130 may be applied in a varietyof ways, and examples of such methods are described in detail in U.S.Pat. No. 6,372,376 to Fronk et al. and may include (1) electrophoreticdeposition, (2) brushing, spraying or spreading, or (3) laminating. Thepresent invention is adaptable for use in continuous manufacturingprocess such as coil coating. Electrophoretically deposited coatings areparticularly advantageous because they can be quickly deposited in anautomated process with little waste, and can be deposited substantiallyuniformly onto substrates 132 having complex and recessed surfaces likethose used to form the reactant flow fields on the working surface(s) ofthe conductive elements. Electrophoretic deposition is a well-knownprocess used to apply polymers to conductive substrates. Whencross-linkable polymers are used, the suspension also includes acatalyst for promoting the cross-linking. Cathodically depositedcoatings are preferred for fuel cell applications, and are deposited bya process wherein positively charged polymer is deposited onto anegatively charged substrate. Cathodic epoxies, acrylics, urethanes andpolyesters are useful with this method of depositing the coating. Otherexamples of suitable polymers include thermoset and thermoplasticresins, such as those disclosed in the U.S. Pat. No. 6,372,376 to Fronk,et al. and the references cited therein. Subsequent baking of the coatedconductive element cures and densifies the coating.

According to another embodiment of the invention, the coating 130 isfirst formed as a discrete film (e.g. by solvent casting, extrusionetc.), and then laminated onto the working surface of the conductiveelement, e.g., by hot rolling. This technique will preferably be used tomake laminated sheet stock from which the conductive elements aresubsequently formed, e.g. as by stamping. In this embodiment, thediscrete film will preferably contain a plasticizer to improve handlingof the film and to provide a coating layer atop the substrate 132 thatis supple enough so that it can be readily shaped, (e.g. stamped)without tearing or disrupting the film when the conductive element isformed as by stamping. To ensure adherence of the coating 130 to thesubstrate 132, the surface of the substrate 132 to which the film isapplied should be pre-cleaned as described above including removing allundesirable surface films (e.g., oil). It is also preferred that thesurface is treated with a conversion coating, roughened, or abraded toroughen the surface for anchoring the polymer film thereto.Fluoropolymers such as polyvinylidene difluoride and a fluoroelastomer,an FPM fluoroelastomer, such as Viton vinylidene fluoridehexafluoropropylene copolymer from DuPont Dow are exemplary, or the likeare useful with this embodiment, and may be used with conventionalplasticizers such as dibutyl phthalate.

According to another embodiment of the invention, the electricallyconductive polymer film 130 is applied to the surface of the metalsubstrate 132 by spraying, brushing or spreading (e.g. with a doctorblade). In this embodiment, a precursor of the coating 130 is formed bydissolving the polymer in a suitable solvent, mixing the conductivefiller particles with the dissolved polymer and applying it as a wetslurry atop the substrate 132. The wet coating is then dried (i.e., thesolvent removed) and cured as needed (e.g., for thermosets). Theconductive particles adhere to the surface by means of the solvent-freepolymer.

A preferred polymer useful with this embodiment comprises apolyamide-imide thermosetting polymer. The polyamide-imide is dissolvedin a solvent comprising a mixture of N-methylpyrrolidone, propyleneglycol and methyl ether acetate. To this solution is added about 21% toabout 23% by weight of a mixture of graphite and carbon black particleswherein the graphite particles range in size from about 5 microns toabout 20 microns and the carbon black particles range in size from about0.5 micron to about 1.5 microns with the smaller carbon black particlesserving to fill the voids between the larger graphite particles andthereby increase the conductivity of the coating compared toall-graphite coatings.

The mix is applied to the substrate 132, dried and cured to provide15-30 micron thick coatings (preferably about 17 microns) having acarbon-graphite content of about 38% by weight. It may be cured slowlyat low temperatures (i.e., <200° C.), or more quickly in a two stepprocess wherein the solvent is first removed by heating for ten minutesat about 150° C. to 175° C. (i.e., dried) followed by higher temperatureheating (250° C. to 400° C.) for various times ranging from about 30seconds to about 900 seconds (15 min), the time being dependent on thetemperature used to cure the polymer.

EXAMPLE 1

An electrically conductive element according to a preferred embodimentof the present invention is prepared by providing stainless steel stripssamples measuring approximately 31 cm×18 cm×0.02 cm. The stainless steelis Type 316L, cold rolled, bright annealed, strip, UNS S31603 inaccordance with ASTM A480 from Arcelor, S.A. located in Grand-Duché,Luxembourg. The steel substrate is pre-cleaned by spraying an alkalinecleaning solution of PARCO®) Clean 1200 (an alkaline cleaner with abuilder constituent comprising potassium hydroxide and tetrapotassiumpyrophosphate) at 5 wt % and 95 wt % deionized water for 10 seconds at71° C. A deionized water rinse is sprayed onto the surface to remove anyresidual alkaline cleaning solution.

An acidic treatment solution is prepared by admixing 3 wt % sulfuricacid solution; 97 wt % deionized water solvent; and 284 ppm of HF. Thesulfuric acid can be purchased as RIDOLENE® 123, available from HenkelSurface Technologies of Madison Heights, Mich., which has a sulfuricacid concentration range of between 30 to 60 wt %, as well as asurfactant. RIDOLENE® 123 also contains up to about 10% surfactant. Thehydrofluoric acid can be purchased from VWR Scientific Products Inc. Theacidic treatment solution is sprayed on the steel substrate surface for10 seconds at 66° C. to activate the surface and remove metal oxides.The steel substrate is then sprayed for about 10 seconds at about 66° C.with a first rinse solution comprising 1 wt % H₂SO₄ and 99 wt %deionized water. The steel substrate is further sprayed for about 10seconds at about 66° C. with a second rinse solution comprising 100 wt %deionized water. The surface of the steel substrate is then dried by apair of squeegee rollers at 26° C., such as 30-90 Shore A Durometerelastomer such as ethylene propylene rubber or polychloroprene.

An electrically-conductive protective polymer coating matrix supplied byAcheson Colloids Company, Port Huron, Mich. as Electrodag® EB-008 isapplied to the stainless steel substrate by spray coating at roomtemperature (26° C.). The stainless steel substrate having a spraycoating of polymer matrix is placed in an oven at 150° C. for 10 minutesto vaporize the solvents and is then placed in an oven at 260° C. for 30minutes to cure the polymeric matrix.

EXAMPLE 2

An electrically conductive element prepared according to an alternatepreferred embodiment of the present invention is prepared by providingthe same stainless steel strips samples as those in Example 1. The steelsubstrate is pre-cleaned with the same alkaline cleaning solution andrinse solution as in Example 1.

An acidic treatment solution is prepared by admixing 3 wt % sulfuricacid solution; 97 wt % deionized water solvent; and 568 ppm of HF. Thesulfuric acid is provided as RIDOLENE® 123, as used in Example 1. Thehydrofluoric acid can be purchased from VWR Scientific Products Inc. Theacidic treatment solution is sprayed on the steel substrate surface for10 seconds at 66° C. to activate the surface and remove metal oxides.The steel substrate is then rinsed, dried, and coated in the same manneras the conductive element described in Example 1.

EXAMPLE 3

An electrically conductive element prepared according to anotheralternate preferred embodiment of the present invention is prepared byproviding the same stainless steel strips samples as those described inExample 1. The steel substrate is pre-cleaned at the same conditionswith the same alkaline cleaning solution and rinse solution as inExample 1.

An acidic treatment solution is prepared by admixing 3 wt % sulfuricacid solution and 97 wt % deionized water solvent. The sulfuric acid isprovided as RIDOLENE® 123, as used in Example 1. The acidic treatmentsolution is sprayed on the steel substrate surface for 10 seconds at 66°C. to activate the surface and remove metal oxides. The steel substrateis then rinsed, dried, and coated in the same manner as the conductiveelement described in Example 1.

Immediately after the application of acidic treatment solution andbefore the protective coating in Examples 1, 2, and 3, the contactresistance measurements of the activated stainless steel samples weremeasured via a testing apparatus as shown in FIG. 6. The testingapparatus comprises a carver press 200 with gold coated platens 202 anda first and second electrically conductive activated carbon paper media204,206 respectively, pressed between a sample 208 and the gold coatedplatens 202. A surface area of 49 cm² was tested using 50 A/cm² currentwhich is applied by a direct current supply. The resistance is measuredusing a four-point method and calculated from measured voltage drops andfrom known applied currents and sample 208 dimensions. The voltage dropis measured across either two diffusion media 204,206 through the sample208 (total contact resistance) or across two points on the sample 208surface 209 (bulk contact resistance). The sample 208 may comprise asingle stainless steel plate or a bipolar plate having two plates 210joined (e.g. brazed) together, which is shown in FIG. 6. In thecircumstance where the bipolar plate is tested, the bulk contactresistance is measured from a midpoint 212 between the two plates 210 toan exterior 214 of either plate 210, to establish a value for a singleplate 210. Contact resistance measurements were measured as milli-Ohmsquare centimeter (mΩ-cm²) with incremental force applied at thefollowing pressures: 200 p.s.i. (1400 kPa), 300 p.s.i. (2000 kPa), and400 p.s.i. (2700 kPa).

It should be noted that the contact resistance of the conductive carbonpaper 204,206 is generally a known value, which can be subtracted fromthe measurement to establish the contact resistance of the metal plate210 only. During testing of the samples, a 1 mm thick Toray carbon paper(commercially available from Toray as TGP-H-0.1T) was used for the firstand second carbon paper media 204,206. However, in many circumstancesthe contact resistance of the conductive paper 204,206 is negligible andadds such a small incremental value to the contact resistance value,that it need not be subtracted. The values referred to herein are forthe interfacial electrical contact resistance of the sample 208 only,and reflect contact resistance values for the metal plate 210 alone.

Results of such testing are shown in FIG. 7 and Table 1. FIG. 7 showsthe contact resistance for the sample in Example 1, as well as Control1, which is an untreated sample of the stainless steel strip asdescribed above in the Examples, as received from the manufacturer. FIG.7 also shows Control 2, which is a similar sample of 316L stainlesssteel; however it was treated with only the known method of cleaningwith an alkaline cleaner, PARCO® 1200 for 10 seconds at 66° C. Thecontact resistance measurements were taken for both the upper firstpaper 204 through the sample 208 and the second lower paper 206 throughthe sample 208, with both values averaged to provide the resultingcontact resistance values indicated below in Table 1 as average bulkcontact resistance values in mΩ-cm² for Examples 1-3 and Controls 1-2.

TABLE 1 Applied Pressure Control 1 Control 2 Example 1 Example 2 Example3 200 p.s.i 637.0 399.4 4.5 5.2 5.9 (1400 kPa) 300 p.s.i. 499.8 277.33.7 4.4 4.9 (2000 kPa) 400 p.s.i. 377.8 158.3 3.4 4.2 4.6 (2700 kPa)

As can be observed from FIG. 7 and Table 1, the samples that weretreated according to the present invention have a significantly reduced(2 orders of magnitude less) contact resistance. Although the prior artcleaning with an alkaline cleaner shows a reduction in contactresistance, the contact resistance still remains impermissibly high foruse as an electrically conductive element. Further, it should be notedthat the treatment times with acidic treatment solution are 10 secondsin Examples 1, 2, and 3, and achieve contact resistance values within adesirable range.

Although, the description of the present invention in the Examples aboveis merely exemplary, it is preferred for conductive elements accordingto the present invention that the contact resistance for theelectrically conductive substrate is less than about 10 mΩ-cm² under acompressive force. Based on the trends exhibited here for contactresistance as a function of compressive force, even a contact resistancecorresponds to a contact resistance measurement with low compressiveforce corresponding to firm, forced engagement (i.e., up to 200 psi)according to the present invention is better than that of the controlsunder significant compressive force (i.e., 300 to 400 psi). Thus, inpreferred embodiments of the present invention, where the compressiveforce is applied at 400 psi applied pressure (i.e., 2700 kPa) asmeasured across the conductive element sample and one piece ofconductive paper, is less than 10 mΩ-cm², and most preferably less than5 mΩ-cm². When the protective coating is applied over the treatedsurface, it is preferred that the overall contact resistance, throughthe conductive protective coating, and treated surface of the metalsubstrate is less than 30 mΩ-cm² under a compressive force applied at apressure of about 400 psi. (2700 kPa) and most preferably below totalcontact resistance of less than about 5 mΩ-cm². Thus, the presentinvention provides a method and composition for treating an electricallyconductive element for a fuel cell that reduces the metal substratecontact resistance to levels that permit the use of previouslyunavailable metals in a shorter duration than was previously feasible.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A method of treating a surface of a stainless steel substrate havingmetal oxide at the surface, the method comprising: contacting thesurface of the stainless steel substrate with an acidic solution havinga pH of from 0 to about 4 comprising one or more halogen ions present insaid solution at a concentration of less than about 1×10⁻³ to about1×10⁻⁶ molarity, wherein said solution reacts with the metal oxide inthe absence of an impressed electrical current, and said anion reactswith said metal oxide to form a metal halide species soluble in saidsolution; and separating said metal halide species and said acidicsolution from the surface to reduce the amount of said metal oxidethereon, wherein said stainless steel substrate is an electricallyconductive element adapted for use in an electrochemical cell, whereinsaid separating to remove said metal oxide reduces electrical resistanceof the surface thereby improving electrical conductivity of the elementin said electrochemical cell.
 2. The method of claim 1, wherein saidcontacting is conducted at a temperature of from about 25° C. to about100° C.
 3. The method of claim 1, wherein said contacting is conductedfor a duration of less than 600 seconds.
 4. The method of claim 1,wherein said separating comprises rinsing the surface with a rinsesolution, thereby removing said acidic solution and said metal halidefrom the surface.
 5. The method of claim 1, wherein said solutioncomprises one or more of water, hydrofluoric acid, sulfuric acid, andnitric acid.
 6. The method of claim 1, wherein said solution has a pH offrom about 2 to about 4, wherein said solution comprises said one ormore halogen ions at a concentration of from about 1×10⁻³ to about1×10⁻⁵ molarity.
 7. The method of claim 1, wherein said metal oxideprovides the source of said metal in said metal halide and said halogenion provides the source of said halide in said metal halide species. 8.The method of claim 1, wherein said halogen ion is selected from thegroup consisting of: F⁻, Cl⁻, Br⁻, I⁻, and mixtures thereof.
 9. Themethod of claim 1, wherein said halogen ion comprises fluorine.
 10. Themethod of claim 1, wherein said solution comprises hydrofluoric acid.11. The method of claim 1, wherein said solution comprises water,sulfuric acid, and said halogen ions comprise fluoride ions that arepresent at a concentration of between about 1×10⁻³ and about 1×10⁻⁵. 12.A method of treating an electrically conductive element for use in anelectrochemical cell comprising stainless steel having metal oxides on asurface, the method comprising: contacting the surface with an acidicsolution having fluoride ions present at a concentration of less thanabout 1×10⁻³ to about 1×10⁻⁶ molarity for reaction with at least aportion of the metal oxides in the absence of an impressed electricalcurrent, wherein said reaction forms a metal fluoride species soluble insaid solution; separating said metal fluoride and said acidic solutionfrom the surface to remove said portion of the metal oxides and toreduce electrical resistance of the surface thereby improving electricalconductivity of the element.
 13. The method of claim 12, wherein saidcontacting is conducted at a temperature of from about 25° C. to about100° C.
 14. The method of claim 12, wherein said contacting is conductedfor a duration less than 600 seconds.
 15. The method of claim 12,wherein said separating comprises rinsing the surface with a rinsesolution, thereby removing said acid solution and said metal fluoridefrom the surface.
 16. The method of claim 12, wherein the metal oxidesprovide the source of said metal in said metal fluoride and saidfluoride ion provides the source of said fluoride in said metalfluoride, whereby the amount of the metal oxides at the surface islessened.
 17. A method of activating a substrate surface of anelectrochemical cell, the method comprising: providing a stainless steelalloy substrate comprising one or more elements of iron, chromium,nickel, and molybdenum, wherein said one or more elements form a metaloxide species; determining an instable region of said metal oxidespecies by thermodynamic phase data; creating a solution for removing ofsaid metal oxide species within said instable region, wherein saidsolution is at a pH and comprises one or more halogen ions at aconcentration of from about 1×10⁻³ to about to about 1×10⁻⁶ molaritysuch that said solution reacts with said metal oxide species within saidinstable region; contacting said solution with the substrate surface toreact said solution with said metal oxide species to form a metal halidespecies soluble in said solution; and separating said metal halidesoluble species from the substrate surface, thereby removing said metaloxide species and activating the substrate surface.
 18. The method ofclaim 17, wherein said determining includes evaluating thermodynamicphase data at conditions including one or more of pH, temperature,potential energy, and presence of one or more halide anions.
 19. Themethod of claim 17, wherein said solution comprises water, sulfuricacid, and said halogen ions comprise fluoride ions that are present at aconcentration of between about 1×10⁻³ and about 1×10⁻⁵, said solutionhas a pH of from about 2 to about 4, and said contacting is performed ata temperature of from about 25° C. to about 100° C.